AC Transit is a public transit agency that operates 131 bus lines throughout the East Bay region of California. The agency also administers the Dumbarton Express lines, but operation of those lines was transferred to MV Transportation on December 19, 2011. AC Transit provides extensive local bus service to 17 cities in Alameda and Contra Costa counties and the city of Milpitas in Santa Clara County. Transbay bus service is also provided to San Francisco in San Francisco County, San Mateo in San Mateo County, and Palo Alto and Santa Clara in Santa Clara County.

Route summary AC Transit bus routes are arranged in six categories (five numbered, one lettered). Since its inception in 1960, AC Transit has used both numbers and letters to distinguish routes. Many Transbay rail lines were inherited from the Key System and converted to bus routes, and AC Transit continued to use letters to identify the routes. Many Transbay routes continue to follow the original Key System alignments to some degree. The number and letter ranges listed below do not mean that all numbers or letters within a range are in use.

Numbered routes operate within the East Bay: 1-99 primarily serve the northwestern part of Contra Costa County and most of Alameda County. 200-299 serve the southern part of Alameda County and Milpitas. 600-699 operate primarily on school days only. 700-799 are Early Bird Express lines, which are replacing early-morning BART service during a seismic retrofit of the Transbay Tube 800-899 are part of the All Nighter network, which provides bus service during owl hours and at times when BART is not operating.

One- and two-digit route numbers are sometimes used as a "base" number for three-digit route numbers. For example, Route 840 is "based" on Route 40.

Lettered routes (A-Z) provide Transbay service (i.e., service across the San Francisco Bay by bridge) between the East Bay and San Francisco, San Mateo, and Santa Clara Counties.

Suffixes (also lettered) are used to distinguish variations in route alignments. Currently, A, B, L, M, S, and X are used. Some suffixes have specific meanings: L denotes Limited stop service, where not all local stops are served. Used on Routes 46L, 72L, and NL. X denotes Express service, where portions of the main route are skipped entirely. Used on Routes NX and NX3.

Supplemental numerical suffixes are used to differentiate route variations for lines that already use a lettered suffix. This is the case with the NX lines listed above.

Note: Routes listed below are current as of 14 June 2026.

Local service

These routes serve various cities, towns, and attractions in Alameda and Contra Costa counties. One route also serves Milpitas in Santa Clara County. Several routes provide deviations on select trips, where a bus serves a particular business or school that is not on the regular route.

Historically, Route 51 has been considered the busiest bus route in the East Bay, connecting the cities of Berkeley, Oakland, and Alameda. However, this route was split into Routes 51A and 51B in March 2010.

In early 2015, following several rounds of community meetings, AC Transit announced a proposal for a radical rerouting and redesignation of many lines throughout their service area due to extra funds made available by the passage of Measure BB in November 2014. Called AC Go, the program looks at redesigning bus lines throughout the system to reflect the changing traveling habits of its residents by providing more frequent service on its busiest lines while rerouting others to better serve adjoining communities.

In August 2025, AC Transit launched Realign, a major overhaul of the bus network that aims to provide more reliable service in the East Bay by simplifying routes and connecting new destinations.AC Transit Realign